1. Field of the Invention
The present invention relates to a method and control system for monitoring exhaust gas conversion efficiency of a catalytic converter communicated to an internal combustion engine which is supplied with a so-call flex fuel which may include first and second fuels present in different concentrations from one refueling to the next.
2. Description of Related Art
U.S. Pat. No. 5,899,062 describes a method and control system for monitoring the efficiency of a catalytic converter communicated to the exhaust manifold of the internal combustion engine. A controller processes signals from pre-catalyst oxygen sensor (HEGO sensor) positioned upstream of the catalytic converter and a post-catalyst oxygen sensor (catalyst monitor sensor known as CMS) positioned downstream of the catalytic converter and determines a ratio which is based on an instantaneous incremental arc length, and/or accumulated series of incremental arc lengths, of the downstream sensor signal divided by the instantaneous incremental arc length, and/or accumulated series of incremental arc lengths, of the upstream sensor signal. The ratio is indicative of the efficiency of the catalytic converter and is compared with subsequently generated ratios to monitor converter efficiency over time. When the ratio exceeds a ratio threshold value, the catalytic converter is deemed to malfunction, and a malfunction indicator light (MIL) is illuminated to alert the vehicle operator.
In practicing the method and control system of the above patent, the arc lengths and corresponding ratio are calculated when predetermined global and local entry conditions are met so as to reduce unwanted variations in the calculation due to factors that are unrelated to catalyst conversion efficiency.
Currently, internal combustion engines of motor vehicles are being adapted to operate on so-called flexible fuels that include gasoline and various percentage blends of gasoline and ethanol because of possible reduction in certain regulated emissions. As a result, there is a need for a method and control system for monitoring the efficiency of a catalytic converter when the internal combustion engine will be operated on such flexible fuels where the concentration of gasoline in the fuel supplied to the engine from the fuel tank may vary from about 90% by volume to as low as about 15% by volume in gasoline/ethanol blended fuel and where the concentration of gasoline in the fuel in the fuel tank may vary from one tank refueling to the next as the motor vehicle is operated.
An object of the present invention is to provide a method and control system for monitoring the exhaust gas conversion efficiency of a catalytic converter that satisfies this need.
The present invention provides pursuant to one embodiment a method and control system for monitoring the performance of a catalytic converter where a ratio indicative of catalytic converter exhaust gas conversion efficiency is determined from upstream and downstream exhaust gas sensor signals and compared to a ratio threshold value (e.g. an index ratio threshold value) that is indicative of a malfunctioning catalytic converter and that is adaptively adjusted in dependence on a fuel blending ratio that reflects the current concentration of at least one of a first fuel and second fuel, such as for example gasoline and ethanol, in the fuel tank. The comparison is used as an indication of whether or not the catalytic converter is malfunctioning.
The present invention is advantageous to adjust the ratio threshold value in dependence on the fuel composition in the fuel in the fuel tank in a manner to provide more accurate monitoring of the catalytic converter under variable fuel blending ratio conditions. Adjustment of the ratio threshold value pursuant to the invention will avoid false activation of a malfunction indicator light (MIL) that otherwise might occur as a result of the effect of a particular fuel blending ratio on signal characteristics of the downstream exhaust gas sensor (e.g. a downstream catalyst monitor sensor).
The above objects and advantages of the present invention will become more readily apparent from the following description taken with the following drawings.